Over the recent years, wireless traffics have continued to rapidly increase, and there have been ever-increasing demands for frequencies as finite resources. As one of means to effectively utilize the frequencies, an examination about a cognitive radio technology to perform communications by cognizing an ambient radio environment is in progress (refer to, e.g., Non-Patent document 1). For example, attentions are focused on white space (or frequency shared) cognitive radio. The white space cognitive radio is defined as a function to perform communications by use of a free space of frequencies corresponding to time, locations, etc. without interfering a system enabled to employ preferentially the respective frequencies. This free space of the frequencies is called a white space. For example, the US FCC (Federal Communications Commission) announced a final rule (Non-Patent document 2) concerning communications-oriented utilization of a TV white space (TVWS) in September of 2010 in U.S.A.
In a white space cognitive radio technology, the system having a priority to use the frequencies is referred to as a prima system or a primary system. A system, which detects and uses the white space of a frequency band allocated to the primary system, is referred to as a second system or a secondary system. In the case of the TVWS, TV broadcasting becomes the primary system. Broad frequency bands such as UHF bands are allocated to the TV broadcastings, in which an actually used frequency, e.g. a physical TV channel, differs depending on a local area. Further, the TVWS has a characteristic of being small in fluctuation in a timewise direction. A database access method adopted in the FCC rule is regarded prospective as a method of detecting such a quasi-static TVWS. The database access method involves storing, in a database configured on a network, radio zone information of the primary system and usable white spaces of the secondary system, the WS being obtained based on a counter primary system interference avoidance standard. The secondary system desiring to use the white space refers to the database with self positional information being used (as query information), thus obtaining a list of free frequencies usable by the secondary system itself. The free frequencies are also referred to as white space frequencies.